An alternative route to enantioenriched α-alkoxyalkylstannanes by stereoselective opening of chiral α-stannylacetals with organometallic reagents

Abstract

α-Tributylstannylacetals derived from chiral C2 symmetry axis diols were reacted with miscellaneous organometallic reagents to give chiral α-oxygenated organotins in high yields. The Lewis acid promoted ring opening of the these chiral α-tributylstannylacetals by organocopper reagents, allyltins or silylenol ethers has been considered to occur mainly according to an anti process (d.r = 7030 to 937), the absolute configuration of the newly created centre being S when the reaction was performed with Me2CuLiBF3 on the α-stannylacetal derived from (2S,4S)-2,4-pentanediol. Of interest is the reverse stereochemical trend obtained using organo-aluminium reagents (d.r. = 3070 to 1585) since it becomes possible to reach selectively the new chiral centre with a preferential R or S configuration starting from the same precursor. The obtained α-alkoxyalkylstannanes can be transmetallated with n-butyllithium (THF, −78°C) to give configurationally stable α-alkoxyalkyllithiums. Furthermore, if desired, the enantiorenriched α-alkoxyalkylstannanes derived from 2,4-pentanediol can be converted into enantioenriched α-hydroxyalkylstannanes (subsequently protected as MOM derivatives) with retention of the configuration at the asymmetric carbon using an appropriate oxidation-β-elimination sequence.